In the field of semiconductor devices, the device density is increased and the device dimension is reduced, continuously. The demand for the packaging or interconnecting techniques in such high density devices is also increased to fit the situation mentioned above. Flat no-leads packages such as QFN (quad-flat no-leads) and DFN (dual-flat no-leads) physically and electrically connect integrated circuits to printed circuit boards. Flat no-leads, also known as Micro-Lead-Frame and SON (small-outline no leads), is a surface-mount technology, one of several package technologies that connect ICs to the surfaces of PCBs without through-holes. Flat no-lead is a near chip scale package plastic encapsulated package made with a planar copper lead frame substrate. Perimeter lands on the package bottom provide electrical connections to the PCB. Flat no-lead packages include an exposed thermal pad to improve heat transfer out of the IC (into the PCB). Heat transfer can be further facilitated by metal vias in the thermal pad. The QFN package is similar to the quad-flat package, and a ball grid array.
Two types of QFN packages are common: air-cavity QFNs, with an air cavity designed into the package, and plastic-molded QFNs with air in the package minimized. Less-expensive plastic-molded QFNs usually limited to applications up to ˜2-3 GHz. It is usually composed of just 2 parts, a plastic compound and copper lead frame, and does not come with a lid. In contrast, the air-cavity QFN is usually made up of 3 parts; a copper lead-frame, plastic-molded body (open, and not sealed), and either a ceramic or plastic lid. It is usually more expensive due to its construction, and can be used for microwave applications up to 20-25 GHz. QFN packages can have a single row of contacts or a double row of contacts. The QFN package is similar to the Quad Flat Package, but the leads do not extend out from the package sides. It is hence difficult to hand-solder a QFN package. A QFN package is very similar to a Ball grid array (BGA), except that the QFN uses landing pads, and solder paste is required to mount a QFN package onto the PCB. A BGA-package does not require any solder as the balls will melt and make contact with the PCB.
Different manufacturers use different names for this package: ML (micro-lead-frame) versus FN (flat no-lead), in addition there are versions with pads on all four sides (quad) and pads on just two sides (dual), and various thickness varying between 0.9-1.0 mm for normal packages and 0.4 mm for extreme thin.
For example, Micro lead-frame package (MLP) is a family of integrated circuit QFN packages, used in surface mounted electronic circuits designs. It is available in 3 versions which are MLPQ (Q stands for quad), MLPM (M stands for micro), and MLPD (D stands for dual). These packages generally have an exposed die attach pad to improve thermal performance. This package is similar to chip scale packages (CSP) in construction. MLPD are designed to provide a footprint-compatible replacement for small-outline integrated circuit (SOIC) packages.
MicroLeadFrame (QFN—Quad Flat No-Lead package) is a near CSP plastic encapsulated package with a copper lead-frame substrate. This package uses perimeter lands on the bottom of the package to provide electrical contact to the printed circuit board. The die attach paddle is exposed on the bottom of the package surface to provide an efficient heat path when soldered directly to the circuit board. This also enables stable ground by use of down bonds or by electrical connection through a conductive die attach material. Based on this package structure, it still has the die attached material formed between the die back and lead frame, and the high thermal resistance comes from the die attached materials itself.
A more recent design variation which allows for higher density connections is the Dual Row MicroLeadFrame (DRMLF) package. This is an MLF package with 2 rows of lands for devices requiring up to 164 I/O. Typical applications include hard disk drives, USB controllers, and Wireless LAN.